1. Field of the Invention
The present invention relates to a thermal printer and, more particularly to a thermal printer having a line thermal head, for performing direct-thermal printing (in which selected portions of heat-sensitive paper are heated by the thermal head, thereby to form characters on the heat-sensitive paper), or a heat-transfer thermal printing (in which selected portions of ink ribbon, which contact to a paper, are heated by the thermal head, thereby to transfer ink of the ribbon onto the paper).
2. Description of the Related Art
A bar-code printer is known which prints bar codes on labels. This printer comprises two side frame plates extending vertically and parallel to each other, a platen roller rotatably supported at both ends by the frame plates, a head frame rockably supported by the frame plates, and a line thermal head secured to the head frame. The bar-code printer further comprises a head-pressing mechanism provided on the frame plates, for pressing the line thermal head onto the platen roller. A sheet of paper can be passed between the line thermal head and the platen roller, so that the thermal head can print characters on the sheet.
Since the thermal head is spaced apart at one end, from the stationary paten roller, while the other end is contacting the platen roller, the printing pressure or contact pressure of the thermal head with respect to the platen roller is uneven in the longitudinal direction of the platen roller, inevitably deteriorating the printing quality. In the conventional thermal printer, to prevent application of an uneven printing pressure to the platen roller, the following measures are taken. The line thermal head has a projection protruding from its middle portion in the axial direction of the head. This projection is connected to the head frame by means of a pivot extending in a direction perpendicular to the axis of the line thermal head. The head is allowed to rotate freely around the pivot, only at its one end.
According to the conventional printer described above, however, the head frame must be large and massive because the head-pressing mechanism is mounted on the head frame. Thus, the bar-code printer will require a relatively large space in which to move the head frame along with the pressing mechanism, thereby making the printer to be large as a whole.
The bar-code printer can be used to provide either a direct-thermal printing or a heat-transfer thermal printing. When it is used for a direct-thermal printing, no ink-ribbon supply device is mounted on the head frame. When it is used for a heat-transfer thermal printing, an ink-ribbon supply device is mounted on the head frame. Apparently, the printing pressure the head applies to recording paper when no ink-ribbon supply device is placed on the head frame is different from the printing pressure the head exerts on recording paper when an ink-ribbon supply device is mounted on the head frame. Needless to say, such a difference in the printing pressure applied to the paper results in a difference in the quality of the bar codes printed by the bar-code printer on the paper.